Fossil discovery: ancient turtle had no shell

Archaeologists have unearthed new proof that the turtle’s most ancient ancestors did not have the species’ most distinctive feature: a shell. The study was published on Wednesday in the scientific journal Nature.

The 228 million-year-old fossil of the shell-less turtle was discovered in the Guizhou Province in south-west China in remarkably intact condition, spanning roughly the size of a double bed. It’s said to be the first owner of a toothless beak which has inspired its snappy new name of Eorhynchochelys sinensis (‘Dawn turtle with a beak from China’). The toothless beak is another characteristic feature of the turtle clan.

To the untrained eye, modern turtles may look like they’ve barely bothered evolving from the prehistoric era, but it turns out the path to contemporary turtle-hood has been a strange, convoluted road that’s more complicated than with other species.

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This latest discovery is one of the oldest proto-turtles ever found; the next one in the evolutionary chain – a beast named Odontochelys which lived 220 million years ago – appeared to have the bottom of the shell covering its underbelly (scientific term: a plastron) but nothing on top to protect its back (a carapace).

Dr Nicholas Fraser, keeper of natural sciences at the National Museum of Scotland in Edinburgh comments that you can begin to see the development of this plastron on the Eorhynchochelys, citing a telltale expansion of the ribs into a flat, rugby ball shape.

Odontochelys, however, did not have a beak, suggesting that the two features evolved separately in different strains of DNA before coming together in the body of one animal – an eccentric process known as ‘mosaic evolution’.

The discovery helps to answer a question that’s long been vexing turtle researchers. Because of the emergence of turtles with different traits that refuted the possibility of a simple, linear evolutionary process, it’s been unclear whether ancient turtles belonged to the same reptilian group as lizards and snakes (diapsids). The two holes found on the skull of the Eorhynchochelys show now that they do.

Shells are an evolutionary boon not just for the physical protection they give to an animal’s body. They also help turtles survive underwater for long periods of time. The shell can store up potassium and magnesium which help neutralise the toxic lactic acid that accumulates when anaerobic respiration (breathing without oxygen) takes place. The lack of a shell suggests that this turtle probably spent most of its time on land or in shallow waters and found its food in mud rather than the sea.

Eorhynchochelys would have been a contemporary of the world’s very earliest dinosaurs, but amazingly it’s not the only animal from the Triassic era that’s mostly still recognizable in its current form; phytosaurs, a distant relative of the crocodile, also stalked Pangea at roughly the same period.

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